Large volume detectors

Transformation of the process diagram into a corresponding simulation flow sheet is illustrated in Figure 6.5b. In principle, all plant elements may be represented by separate submodels. For practical applications, though, it is typically sufficient to take into account only a time delay as well as the dispersion of the peaks until they enter the column. This can be achieved by a pipe flow model that includes axial dispersion. Large volume detectors (including some connecting pipe-s) can be approximated, if required, by a stirred tank model. 

The maximum allowable dispersion will include contributions from all the different dispersion sources. Furthermore, the analyst may frequently be required to place a large volume of sample on the column to accommodate the specific nature of the sample. The peak spreading resulting from the use of the maximum possible sample volume is likely to reach the permissible dispersion limit. It follows that the dispersion that takes place in the connecting tubes, sensor volume and other parts of the detector must be reduced to the absolute minimum and, if possible, kept to less than 10% of that permissible (i.c.,1 % of the column variance) to allow large sample volumes to be used when necessary. 

Microwave movement detectors utilize the principle of the Doppler effect on high-frequency low-power radio waves. These units are moderate in cost and suitable for large-volume coverage. Microwaves, however, penetrate certain materials easily, such as plasterboard, and careful siting is required to avoid false alarms. 

The one major difficulty with use of a flow cell is that the combination of pipe cleaner fibers and a large volume of mother liquor around the crystid greatly increases the background scatter of X-rays, particularly in the 3 A resolution region. For diffractometric data collection—and, probably, for area detector work as well—this problem is not serious, but... 

Figure 3.26 — Schematic view of large volume wall-jet detector (A) inlet (B) electrode array (C) counter-electrode (D) referaice electrode (E) outlet. Also shown (right) bottom view of the electrode array. (Reproduced from [191] with permission of the American Chemical Society).

A steady pressure should be maintained within the flow cell when performing the detector linearity test. Filling the flow cell with the test solution manually with a syringe sometimes leads to pressure fluctuations and hence unstable readings. An alternative is to use the pump to deliver test solution to the flow cell. However, this option requires a large volume of test solution. 

Cl) samples were irradiated in a reactor with thermal neutron flux of 6 X 1013 neutrons/cm2/sec for 2 secs and counted with a large volume Ge(Li) detector. 3 samples of 10 mg each were used. For longer lived isotopes (Cu, Sn, Co, Ag, Au, and Sb) samples were irradiated for 1 hr, cooled for 7 days, diss. in HN03-HF and made up to standard volume for counting. Pure metal foil standards were used for Cu, Sn, Co, Au pure NH4C1 was used for Cl and pure Sb3 for Sb. In calculated from intensity of 116mIn, 1.26 Mev 7 relative to 64Cu 1.34 Mev 7. 

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